During well operations, drilling fluids can be lost into the surrounding formation. To prevent this, the fluid is frequently modified such that a small amount of the fluid and solids contained therein form a coating on the wellbore surface (often referred to as a “filter cake”). After the completion of drilling operations, the coating or filter cake is typically removed, and production from the formation can proceed. The process used to remove the filter cake can also be used to remove other types of damage or debris from the wellbore prior to beginning production.
“Stimulation” refers to processes that can be used to increase the net permeability of a formation or reservoir. Some exemplary known stimulation techniques include: (1) injection of chemicals into the wellbore to react with and dissolve conditions limiting production (e.g., scale, filter cake); (2) injection of chemicals through the wellbore and into the formation to react with and dissolve portions of the formation, or to create alternative flow paths for recoverable hydrocarbons; and (3) injection of chemicals through the wellbore and into the formation at pressures sufficient to cause fractures in the formation, thereby creating a flow channels through which hydrocarbons can more readily move from the formation into the wellbore.
In particular, methods to enhance the productivity of hydrocarbon wells by removing near-wellbore formation damage or by creating alternate flow paths by fracturing and/or dissolving small portions of the formation at the fracture face are respectively known as “matrix acidizing,” and “acid fracturing.” In general, acids, or acid-based fluids are useful in this regard due to their ability to dissolve both formation minerals (e.g., calcium carbonate) and contaminants (e.g., drilling fluid coating the wellbore or that has penetrated into the formation) introduced into the wellbore/formation during drilling or remedial operations.
The removal of filter cake and scale deposits and fluid placement are key concerns in well completion operations. Known prior art techniques involve multiple step processes, which can be costly and time consuming. For example, in one typical prior art application, during completion operations, the process of treating a wellbore may include the repeated addition of an additive (such as an acid or a diverter); followed by the addition of a spacer, and wherein the addition steps may be repeated until all of the required treatments have been finished. The use of high activity acids, such as hydrochloric acid, may also require the use of methods and equipment specific to matrix acidizing, as opposed to other types of injection, thus requiring specific equipment. Matrix acidizing, as described above, is a stimulation method known solely for the purpose of enhancing productivity.
One of the difficulties encountered with traditional matrix acidizing in rig operations is that the formation, particularly high permeability formations, encounter severe circulation loss after the well is acidized using traditional compositions. Thus, removal of the filter cake in highly permeable formations using matrix acidizing techniques can alleviate filter cake issues while at the same time creating a new problem that must be separately addressed. It is desirable to avoid any lost circulation during stimulation of the well. If there is circulation loss during stimulation of the well, typically, rig operations must be stopped, and separate measures must be taken to stop the circulation loss. Loss circulation is typically only a problem when the rig is onsite and the drill pipe is used to place acid into the wellbore as drilling rig pumps typically cannot supply enough fluid to the wellbore if the losses to the formulation are large.
The use of viscoelastic surfactants (“VES”) is known in the petroleum industry for providing controllable fluid viscosity that can be easily broken by hydrocarbons, and which leaves little or no residue after treatment. The exact mechanism of viscoelastic surfactants is understood to involve the formation of rod like micelles upon a change in the environment, such as the pH, salinity, temperature, or other property or condition of the environment. With certain cationic VES fluids, it is typical for the VES to be consumed during the operation such that viscosity of the transport fluid is only temporarily affected, at which point the viscosity modification effects of the VES are lost, VES fluids suffer the drawback, however, of being unable to prevent the fluid loss problems noted above with respect to matrix acidization. For this reason, the VES fluids typically must be combined with fluid loss control agents, such as magnesium oxide, where the particles of the fluid loss control agent appear to associate with the VES micelles to limit the VES fluid flow into the porous media of the formation. Common fluid loss control agents for use with VES are solid particulates.
In oil production operations, it is at times necessary to both remove filter cake and prevent or minimize fluid losses, especially during well stimulation, particularly when using a drill pipe for introducing the well stimulation fluids. Typically, separate fluids are required for fluid loss prevention and for the removal of filter cake. Thus, it would be advantageous to use a single bimodal composition capable to both prevent fluid loss and remove filter cake.